In recent years, for image forming apparatuses such as copiers and printers, there has been a demand to form high-quality images at high speed, and accordingly, image forming apparatuses which expose a photosensitive member to light by outputting a plurality of laser beams (light beams) from a plurality of light-emitting devices have been adopted. Image forming apparatuses have achieved high-quality image formation by increasing the resolution to, for example, 2400 dpi and have achieved high-speed image formation by forming an electrostatic latent image through irradiation of a photosensitive member with a plurality of laser beams (for example, 16 beams) in one scan.
However, when the resolution is, for example, 2400 dpi, the intervals between laser beams in a rotational direction of a photosensitive member is 10.5 μm. When 16 laser beams are used to scan a photosensitive member, a range of one scan in the rotational direction of the photosensitive member is expressed by the following equation, 10.5 (μm)×16=168 μm, and the resolution in one scan period is about 25.4 (mm)/168 (μm)≈150 dpi. Namely, the resolution in the scan period is 150 dpi, and the spatial frequency in one scan is such a frequency as to be visually identifiable, and hence generation of moiré may occur due to a strip-shaped region in one scan and a screen.
For this reason, there has been proposed an image forming method which has a fast mode in which an image is formed by exposing a photosensitive member to light with, for example, 16 beams from all light-emitting devices, and a high-quality mode in which an image is formed by exposing the photosensitive member to light with, for example, 12 beams from a reduced number of light-emitting devices, and the modes are switched according to situations.
In image forming apparatuses, however, the relative positions of optical paths of lasers and optical lenses and mirrors are conventionally adjusted during assembly of the image forming apparatuses before shipment so that the shapes and sizes of spots of laser beams guided onto a photosensitive member can satisfy product specifications.
FIG. 17 is a view showing an image corresponding to an electrostatic latent image formed on a photosensitive member using a plurality of laser beams passing through an area near the center of a lens in a direction vertical to a scanning direction of laser beams and a direction of an optical axis of the lens in an image forming apparatus. FIG. 18 is a view showing an image corresponding to an electrostatic latent image formed on a photosensitive member using a plurality of laser beams including light beams passing through an edge of a lens in a direction vertical to a scanning direction of laser beams and a direction of an optical axis of the lens in an image forming apparatus.
As shown in FIG. 17, for example, in a four-beam scan system, when all laser beams pass through an area near the center of a lens to make an adjustment so that the aberration of the lens can be small, spots 1500 to 1503 of respective laser beams on a photosensitive member are in focus as indicated by (a) in FIG. 17. In this case, as indicated by (b) in FIG. 17, the light quantities of all the laser beams are uniform, and hence a uniform image is formed in one scan as indicated by (c) in FIG. 17. Thus, by making an adjustment so that all laser beams can pass through an area near the center of a lens so as to make lens aberration small, pitch variations in a sub-scanning direction are reduced, and interference between pitch variations and a screen is suppressed.
In reality, however, since an fθ lens and a cylindrical lens have a manufacturing error or a placement error, it is difficult to match the optical axes of laser beams and a generating line of a lens with accuracy. Thus, for example, as shown in FIG. 18, when laser beams output from LDn to LDn-3, which are light-emitting devices, pass through areas successively apart from the center of a lens, aberration gradually increases. As indicated by (a) in FIG. 18, with respect to a spot 1603 of the laser beam from the LDn, a spot 1602 of the laser beam from the LDn-1, a spot 1601 of the laser beam from the LDn-2, and a spot 1600 of the laser beam from the LDn-3, successively increase in spot diameter on a photosensitive member.
In this case, as indicated by (b) in FIG. 18, the light quantity of a laser beam decreases from the LDn to the LDn-3 in this order, and as a result, as indicated by (c) in FIG. 18, a nonuniform image is formed even in one scan. FIG. 19 shows an example in which the density of an image periodically changes in a sub-scanning direction (a rotational direction of a photosensitive member). Due to interference between such image density variations in the sub-scanning direction and a screen, generation of moiré occurs, leading to a problem of bringing about degradation in the quality of an output image.
To solve this problem, for example, there has been proposed a light scanning apparatus having an adjustment device that moves a lens so as to adjust the positions at which a plurality of laser beams falls upon the lens (see PTL (Patent Literature) 1).